In recent years, many efforts have been made to develop an optical recording/reproduction method, apparatus and medium which can satisfy various requirements including high density, large capacity, high access speed, and high recording/reproduction speed.
Of various optical recording/reproduction methods, the magnetooptical recording/reproduction method is most attractive due to its unique advantages in that information can be erased after it is used, and new information can be recorded.
A recording medium used in the magnetooptical recording/reproduction method has a perpendicular magnetic anisotropy layer or layers as a recording layer. The magnetic layer comprises, for example, amorphous GdFe GdCo, GdFeCo, TbFe, TbCo, TbFeCo, and the like. Concentrical or spiral tracks are normally formed on the recording layer, and information is recorded on the tracks. In this specification, one of the "upward" and "downward" directions of the magnetization with respect to a film surface is defined as an "A direction", and the other one is defined as a "non-A direction". Information to be recorded is binarized in advance, and is recorded by two signals, i.e., a bit (B.sub.1) having an "A-directed" magnetization, and a bit (B.sub.0) having a "non-A-directed" magnetization. These bits B.sub.1 and B.sub.0 correspond to "1" and "0" levels of a digital signal. The direction of magnetization of the recording tracks can be aligned in the "non-A direction" by applying a strong bias field. This processing is called "initialization". Thereafter, a bit (B.sub.1) having an "A-directed" magnetization is formed on the tracks.
The principle of bit formation will be described below with reference to FIG. 1.
In the bit formation, a characteristic feature of laser, i.e., excellent coherence in space and time, is effectively used to focus a beam into a spot as small as the diffraction limit determined by the wavelength of the laser light. The focused light is radiated onto the track surface to write information by producing bits less than 1 .mu.m in diameter on the recording layer. In the optical recording, a recording density up to 10.sup.8 bit/cm.sup.2 can be theoretically attained, since a laser beam can be concentrated into a spot with a size as small as its wavelength.
As shown in FIG. 1, in the magnetooptical recording, a laser beam L is focused onto a recording layer 1 to heat it, while a bias field Hb is externally applied to the heated portion in the direction opposite to the initialized direction. A coercivity (denoted Hc herein) of the locally heated portion is decreased below the bias field Hb. As a result, the direction of magnetization of that portion is aligned in the direction of the bias field Hb. In this way, reversely magnetized bits are formed.
Ferromagnetic and ferrimagnetic materials differ in the temperature dependencies of the magnetization and Hc. Ferromagnetic materials have Hc which decreases around the Curie temperature and allow information recording based on this phenomenon. Thus, information recording in ferromagnetic materials is referred to as Tc recording (Curie temperature recording).
On the other hand, ferrimagnetic materials have a compensation temperature, below the Curie temperature, at which magnetization M becomes zero. The Hc abruptly increases around this temperature and hence abruptly decreases outside this temperature. The decreased Hc is canceled by a relatively weak bias field Hb. Namely, recording is enabled. This process is called T.sub.comp. recording (compensation point recording).
In this case, however, there is no need to adhere to the Curie point or temperatures therearound, and the compensation temperature In other words, if a bias field Hb capable of canceling a decreased Hc is applied to a magnetic material having the decreased Hc at a predetermined temperature higher than a room temperature, recording is enabled.
The principle of reproduction will be described below with reference to FIG. 2.
FIG. 2 illustrates the principle of information reproduction based on the magnetooptical effect. Light is an electromagnetic wave with an electromagnetic-field vector normally emanating in all directions in a plane perpendicular to the light path. When light is converted to linearly polarized beams L.sub.P and radiated onto a recording layer 1, it is reflected by the surface or passes through the recording layer 1. At this time, the plane of polarization rotates according to the direction of magnetization (M). This phenomenon is called the magnetic Kerr effect or magnetic Faraday effect.
For example, if the plane of polarization of the reflected light rotates through .theta..sub.k degrees for the "A-directed" magnetization, it rotates through -.theta..sub.k degrees for the "non-A-directed" magnetization. Therefore, when the axis of an optical analyzer (polarizer) is set perpendicular to the plane inclined at -.theta..sub.k, the light reflected by "non-A-direction" magnetized bit B.sub.0 cannot pass through the analyzer. On the contrary, a product of (sin 2.theta..sub.k).sup.2 and the light reflected by a bit B.sub.1 magnetized along the "A direction" passes through the analyzer and becomes incident on a detector (photoelectric conversion means). As a result, the bit B.sub.1 magnetized along the "A direction" looks brighter than the bit B.sub.0 magnetized along the "non-A direction", and the detector produces a stronger electrical signal for the bit B.sub.1. The electrical signal from the detector is modulated in accordance with the recorded information, thus reproducing the information.
In order to re-use a recorded medium, (i) the medium must be re-initialized by an initializing device, or (ii) an erase head having the same arrangement as a recording head must be added to a recording apparatus, or (iii) as preliminary processing, recorded information must be erased using a recording apparatus or an erasing apparatus. Therefore, in the conventional magnetooptical recording method, it is impossible to perform an over-write operation, which can properly record new information regardless of the presence/absence of recorded information.
If the direction of a bias field Hb can be desirably modulated between the "A-direction" and "non-A direction", an over-write operation is possible. However, it is impossible to modulate the bias field Hb at high speed. For example, if the bias field Hb comprises a permanent magnet, the direction of the magnet must be mechanically reversed. However, it is impossible to reverse the direction of the magnet at high speed. Even when the bias field Hb comprises an electromagnet, it is also impossible to modulate the direction of a large-capacity current at high speed.
However, according to remarkable technical developments, a magnetooptical recording method capable of performing an over-write operation by modulating only an intensity of light to be radiated in accordance with binary information to be recorded without turning on/off the bias field Hb or without modulating the direction of the bias field Hb, an over-write capable magnetooptical recording medium used in this method, and an over-write capable recording apparatus used in this method were invented and filed as a patent application (U.S. Ser. No. 453,255 filed on Dec. 20, 1989). The basic invention disclosed in the above-mentioned patent application will be described below.
One of the characteristic features of the basic invention is to use a magnetooptical recording medium comprising a multilayered perpendicular magnetic anisotropy film having at least a two-layered structure including a recording layer (first layer) and a reference layer (second layer). Information is recorded in the first layer (in some cases, also in the second layer) by a bit having an "A-directed" magnetization, and a bit having a "non-A-directed" magnetization.
An over-write method according to the basic invention comprises:
(a) moving a recording medium;
(b) applying an initial field Hini. to align a direction of magnetization of only the second layer in the "A direction" while the direction of magnetization of the first layer is left unchanged before recording;
(c) radiating a laser beam on the medium;
(d) pulse-modulating the beam intensity i accordance with binary information to be recorded;
(e) applying a bias field to the radiated portion when the beam is radiated; and
(f) forming one of a bit having the "A-directed" magnetization and a bit having the "non-A-directed" magnetization when the intensity of the pulse-modulated beam is at high level, and forming the other bit when the beam intensity is at low level.
When recording is performed, the basic invention employs, for example, an over-write capable magnetooptical recording apparatus comprising:
(a) means for moving a magnetooptical recording medium;
(b) initial field Hini. apply means;
(c) a laser beam source;
(d) means for pulse-modulating a beam intensity in accordance with binary information to be recorded to obtain high level that provides, to the medium, a temperature suitable for forming one of a bit having an "A-directed" magnetization and a bit having a "non-A-directed" magnetization, and to obtain low level that provides, to the medium, a temperature suitable for forming the other bit; and
(e) bias field apply means which can be commonly used as the initial field apply means.
In the basic invention, a laser beam is pulse-modulated according to information to be recorded. This procedure itself has been performed in the conventional magnetooptical recording method, and a means for pulse-modulating the beam intensity on the basis of binary information to be recorded is a known means. For example, see "THE BELL SYSTEM TECHNICAL JOURNAL, Vol. 62 (1983), pp. 1923-1936 for further details. Therefore, the modulating means is available by partially modifying the conventional beam modulating means if required high and low levels of the beam intensity are given. Such a modification would be easy for those who are skilled in the art if high and low levels of the beam intensity are given
Another characteristic feature of the basic invention lies in high and low levels of the beam intensity. More specifically, when the beam intensity is at high level, "A-directed" magnetization of a reference layer (second layer) is reversed to the "non-A direction" by means of a bias field (Hb), and a bit having the "non-A-directed" [or "A-directed"] magnetization is thus formed in a recording layer (first layer) by means of the "non-A-directed" magnetization of the second layer. When the beam intensity is at low level, a bit having the "A-directed" [or "non-A-directed"] magnetization is formed in the first layer by means of the "A-directed" magnetization of the second layer.
In this specification, if expressions ooo [or .DELTA..DELTA..DELTA.] appear, ooo outside the parentheses in the first expression corresponds to ooo in the subsequent expressions ooo [or .DELTA..DELTA..DELTA.], and vice versa.
As is well known, even if recording is not performed, a laser beam is often turned on at first low level in order to, for example, access a predetermined recording position on the medium. When the laser beam is also used for reproduction, the laser beam is often turned on at an intensity of the first low level. In this invention, the intensity of the laser beam may be set at this first low level. However, level for forming a bit is second low level higher than the first low level. Therefore, the output waveform of the laser beam of the basic invention is as shown in FIG. 3.
Although not described in the specification of the basic invention, a recording beam need not always be a single beam but may be two proximity beams in the basic invention. More specifically, a first beam may be used as a low-level laser beam (erasing beam) which is not modulated in principle, and a second beam may be used as a high-level laser beam (writing beam) which is modulated in accordance with information. In this case, the second beam is pulse-modulated between high level and base level (equal to or lower than low level, and its output may be zero). In this case, an output waveform is as shown in FIG. 4.
A medium used in the basic invention is roughly classified into a first or second category. In either category, a recording medium has a multilayered structure including a recording layer (first layer) and a reference layer (second layer), as shown in FIG. 5.
The first layer is the recording layer, which exhibits a high coercivity at a room temperature, and has a low magnetization reversing temperature. The second layer is the reference layer, which exhibits a relatively lower coercivity at a room temperature and has a higher magnetization reversing temperature than those of the first layer. Each of the first and second layers may comprise a multilayered structure If necessary, a third layer may be interposed between the first and second layers. In addition, a clear boundary between the first and second layers need not be formed, and one layer can be gradually converted into the other layer.
In the first category, when the coercivity of the recording layer is represented by H.sub.c1 ; that of the reference layer, H.sub.c2 ; a Curie temperature of the first layer, T.sub.c1 ; that of the second layer, T.sub.c2 ; a room temperature, T.sub.R ; a temperature of the recording medium obtained when a low-level laser beam is radiated, T.sub.L ; that obtained when a high-level laser beam is radiated, T.sub.H ; a coupling field applied to the first layer, H.sub.D1 ; and a coupling field applied to the second layer, H.sub.D2, the recording medium satisfies Formula 1 below, and satisfies Formulas 2 to 5 at the room temperature: EQU T.sub.R &lt;T.sub.c1 .apprxeq.T.sub.L &lt;T.sub.c2 .apprxeq.T.sub.H 1 EQU H.sub.c1 &gt;H.sub.c2 +.vertline.H.sub.D1 .-+.H.sub.D2 .vertline.2 EQU H.sub.c1 &gt;H.sub.D1 3 EQU H.sub.c2 &gt;H.sub.D2 4 EQU H.sub.c2 +H.sub.D2 &lt;.vertline.Hini..vertline.&lt;H.sub.c1 .+-.H.sub.D1 5
In the above formulas, symbol ".apprxeq." means "equal to" or "substantially equal to". In addition, of double signs .+-. and .-+., the upper sign corresponds to an A (antiparallel) type medium, and the lower sign corresponds to a P (parallel) type medium (these media will be described later). Note that a ferromagnetic medium belongs to a P type.
The relationship between a coercivity and a temperature is shown in FIG. 6. A first curve represents the characteristics of the first layer, and a second curve represents those of the second layer.
Therefore, when an initial field (Hini.) is applied to this recording medium at the room temperature, the direction of magnetization of only the reference layer (second layer) is reversed without reversing that of the recording layer (first layer) according to Formula 5. When the initial field (Hini.) is applied to the medium before recording, only the second layer can be magnetized in the "A direction" (in the drawings, the "A direction" is indicated by an upward arrow , and the "non-A direction" is indicated by a downward arrow for the sake of simplicity). If the initial field Hini. becomes zero, the direction of magnetization of the second layer can be left unchanged without being re-reversed according to Formula 4.
FIG. 7 schematically shows a state wherein only the second layer is magnetized in the "A direction" immediately before recording. The direction of magnetization * in the first layer represents previously recorded information. FIG. 8 illustrates a direction of magnetization when a high-level laser beam is radiated on the medium shown in FIG. 7, and a bit whose direction of magnetization of the first layer can be disregarded is indicated by X.
In a first condition, a high-level laser beam is radiated to increase a medium temperature to T.sub.H. Since T.sub.H is higher than the Curie temperature T.sub.c1, the magnetization of the recording layer (first layer) disappears. In addition, since T.sub.H is near the Curie temperature T.sub.c2, the magnetization of the reference layer (second layer) also disappears completely or almost completely. The bias field (Hb) in the "A direction" or "non-A direction" is applied to the medium in accordance with the type of medium. The bias field (Hb) can be a stray field from the medium itself. For the sake of simplicity, assume that the bias field (Hb) in the "non-A direction" is applied to the medium. Since the medium is moving, a given irradiated portion is immediately separated apart from the laser beam, and is cooled. When the medium temperature is decreased under the presence of Hb, the direction of magnetization of the second layer is reversed to the "non-A direction" based on Hb (Condition 2.sub.H).
When the medium is further cooled and the medium temperature is decreased slightly below T.sub.c1, Condition 3.sub.H is established, and magnetization of the first layer appears again. In this case, the direction of magnetization of the first layer is influenced by that of the second layer due to a magnetic coupling (exchange coupling) force. As a result, magnetization (the P type medium) or (the A type medium) is formed according to the type of medium.
A change in condition caused by the high-level laser beam will be called a high-temperature cycle herein.
Referring to FIG. 9, a low-level laser beam is radiated to increase the medium temperature to T.sub.L, thus establishing Condition 2.sub.L. Since T.sub.L is near the Curie temperature T.sub.c1, the magnetization of the first layer disappears completely or almost completely. However, since T.sub.L is lower than the Curie temperature T.sub.c2, the magnetization of the second layer does not disappear.
Although the bias field (Hb) is unnecessary, it cannot be turned on or off at high speed (within a short period of time). Therefore, the bias field (Hb) in the high-temperature cycle is left on.
However, since the H.sub.c2 is kept high, the magnetization of the second layer will not be reversed by Hb. Since the medium is moving, a given irradiated portion is immediately separated apart from the laser beam, and is cooled. As cooling progresses, Condition 3.sub.L is established, and the magnetization of the first layer appears again. The direction of magnetization appearing in this case is influenced by that of the second layer due to the magnetic coupling force. As a result, (P type) or (A type) magnetization appears according to the type of medium.
A change in condition caused by the low-level laser beam will be called a low-temperature cycle herein.
As described above, bits having either magnetization or , which are opposite to each other, are formed in the high- and low-temperature cycles regardless of the direction of magnetization of the first layer. More specifically, an over-write operation is enabled by pulse-modulating the laser beam between high level (high-temperature cycle) and low level (low-temperature cycle) in accordance with information to be recorded. This is represented in FIG. 10.
In the above description, both the first and second layers have no compensation temperature T.sub.comp. between the room temperature and the Curie temperature. However, when the compensation temperature T.sub.comp. is present, if the medium temperature exceeds it, the direction of magnetization is reversed, and a change in direction differs depending on A and P types. In addition, the direction of the bias field Hb is opposite to the direction .dwnarw. in the above description at the room temperature
A recording medium normally has a disk shape, and is rotated during recording. For this reason, a recorded portion (bit) is influenced again by the initial field Hini. during one revolution. As a result, the direction of magnetization of the reference layer (second layer) is aligned in the original "A direction" . However, at the room temperature, the magnetization of the second layer can no longer influence that of the recording layer (first layer), and the recorded information can be held.
If linearly polarized light is radiated on the first layer, since light reflected thereby includes information, the information can be reproduced as in the conventional magnetooptical recording medium. In addition, a method of transferring information in the first layer to the second layer aligned in the original "A direction" by applying a reproduction field H.sub.R before reproduction or a method of naturally transferring information in the first layer to the second layer as soon as the influence of Hini. disappears without applying the reproduction field H.sub.R is also available depending on composition designs of the first and second layers. In this case, information may be reproduced from the second layer.
A perpendicular magnetic anistotroy film constituting each of the recording layer (first layer) and the reference layer (second layer) is selected from the group consisting of ferromagnetic and ferrimagnetic materials having no compensation temperature and having a Curie temperature, and an amorphous or crystalline ferrimagnetic material having both the compensation temperature and the Curie temperature.
The first category which utilizes the Curie temperature as the magnetization reversing temperature has been described. In contrast to this, the second category utilizes decreased H.sub.c at a predetermined temperature higher than the room temperature. In a medium of the second category, substantially the same description as the first category can be applied except that a temperature T.sub.s1 at which the recording layer (first layer) is magnetically coupled to the reference layer (second layer) is used in place of T.sub.c1 in the first category, and a temperature T.sub.s1 at which the direction of magnetization of the second layer is reversed by Hb is used in place of T.sub.c2.
In the second category, when the coercivity of the first layer is represented by H.sub.c1 ; that of the second layer, H.sub.c2 ; a temperature at which the first layer is magnetically coupled to the second layer, T.sub.s1 ; a temperature at which the magnetization of the second layer is reversed by Hb, T.sub.s2 ; a room temperature, T.sub.R ; a medium temperature obtained when a low-level laser beam is radiated, T.sub.L ; that obtained when a high-level laser beam is radiated, T.sub.H ; a coupling field applied to the first layer, H.sub.D1 ; and a coupling field applied to the second layer, H.sub.D2, the recording medium satisfies Formula 6 below, and satisfies Formulas 7 to 10 at the room temperature: EQU T.sub.R &lt;T.sub.s1 .apprxeq.T.sub.L&lt;T.sub.s2 .apprxeq.T.sub.H 6 EQU H.sub.c1 &gt;H.sub.c2 +.vertline.H.sub.D1 .-+.H.sub.D2 .vertline.7 EQU H.sub.c1 &gt;H.sub.D1 8 EQU H.sub.c2 &gt;H.sub.D2 9 EQU H.sub.c2 +H.sub.D2 &lt;.vertline.Hini..vertline.&lt;H.sub.c1 .+-.H.sub.D1 10
In the above formulas, of double signs .+-. and .-+., the upper sign corresponds to an A type medium, and the lower sign corresponds to a P type medium (these media will be described later).
Referring to FIG. 11, in the second category, when the medium is at the high temperature T.sub.H, the magnetization of the second layer does not disappear, but is sufficiently weak. The magnetization of the first layer disappears, or is sufficiently weak. Even if sufficiently weak magnetization is left in both the first and second layers, the bias field Hb is sufficiently large, and forces the direction of magnetization of the second layer and that of the first layer in some cases to follow that of the Hb, thus establishing Condition 2.sub.H.
Thereafter, the second layer influences the first layer via .sigma..sub.w immediately, or when cooling progresses after radiation of the laser beam is stopped and the medium temperature is decreased below T.sub.H, or when the irradiated portion is away from Hb, thereby aligning the direction of magnetization of the first layer in a stable direction. As a result, Condition 3.sub.H is established. When the magnetization of the first layer originally has a stable direction, it is left unchanged.
Referring to FIG. 12, when the medium is at the low temperature T.sub.L, both the first and second layers do not lose their magnetization. However, the magnetization of the first layer is sufficiently weak. Therefore, the direction of magnetization of the first layer is influenced, via .sigma..sub.w, by the magnetization of the second layer which is more largely influenced by Hb. In this case, since the second layer has sufficient magnetization, its magnetization will not be reversed by Hb. As a result, Condition 3.sub.L is established regardless of Hb.
In the above description, both the first and second layers have no compensation temperature T.sub.comp. between the room temperature and the Curie temperature. When the compensation temperature T.sub.comp. is present, a more complex situation obtains as described above, and the direction of the bias field is opposite to the direction at the room temperature. In both the first and second categories, the recording medium is preferably constituted by the recording layer (first layer) and the reference layer (second layer) each of which comprises an amorphous ferrimagnetic material selected from transition metal (e.g., Fe, Co)--heavy rare earth metal (e.g., Gd, Tb, Dy, and the like) alloy compositions.
When the materials of both the first and second layers are selected from the transition metal-heavy rare earth metal alloy compositions, the direction and level of magnetization appearing outside the alloys are determined by the relationship between the direction and level of spin of transition metal (to be abbreviated to as TM hereinafter) atoms, and those of heavy rare earth metal (to be abbreviated to as RE hereinafter) atoms inside the alloys. For example, the direction and level of TM spin are represented by a dotted vector , those of RE spin are represented by a solid vector .uparw., and the direction and level of magnetization of the entire alloy are represented by a double-solid vector . In this case, the vector is expressed as a sum of the vectors and .uparw.. However, in the alloy, the vectors and .uparw. are directed in the opposite directions due to the mutual effect of the TM spin and the RE spin. Therefore, when strengths of these vectors are equal to each other, the sum of and .uparw. or the sum of .dwnarw. and is zero (i.e., the level of magnetization appearing outside the alloy becomes zero). The alloy composition making the sum of vectors zero is called a compensation composition. When the alloy has another composition, it has a strength equal to a difference between the strengths of the two spins, and has a vector ( or ) having a direction equal to that of the larger vector. Magnetization of this vector appears outside the alloy. For example, appears as , and and appears as .
When one of the strengths of the vectors of the RE and TM spins is larger than the other, the alloy composition is referred to as "oo rich" named after the larger spin name (e.g., RE rich).
Both the first and second layers can be classified into TM rich and RE rich compositions. Therefore, when the composition of the first layer is plotted along the ordinate and that of the second layer is plotted along the abscissa, the types of media as a whole of the basic invention can be classified into four quadrants, as shown in FIG. 13. The P type medium described above belongs to Quadrants I and III, and the A type medium belongs to Quadrants II and IV. Note that the intersection of the abscissa and the ordinate represents the compensation composition of the two layers.
In view of a change in coercivity against a change in temperature, a given alloy composition has characteristics wherein the coercivity temporarily increases infinitely and then abruptly decreases before a temperature reaches the Curie temperature (at which the coercivity is zero) The temperature corresponding to the infinite coercivity is called a compensation temperature (T.sub.comp.). No compensation temperature is present between the room temperature and the Curie temperature in the TM rich alloy composition. A compensation temperature below the room temperature is irrelevant in the magnetooptical recording, and hence, it is assumed in this specification that the compensation temperature is present between the room temperature and the Curie temperature.
If the first and second layers are classified in view of the presence/absence of the compensation temperature, the recording medium can be classified into four types. A medium in Quadrant I includes all the four types of media. FIGS. 14A, 14B, 14C, and 14D show the relationship between the coercivity and temperature of these four types of media.
When the recording layer (first layer) and the reference layer (second layer) are classified in view of their RE or TM rich characteristics and in view of the presence/absence of the compensation temperature, recording media can be classified into the following nine classes.
TABLE 1 ______________________________________ Class Type ______________________________________ Quadrant I (P type) First Layer: Second Layer: RE Rich RE Rich ______________________________________ 1 T.sub.comp. T.sub.comp. 1 2 No T.sub.comp. T.sub.comp. 2 3 T.sub.comp. No T.sub.comp. 3 4 No T.sub.comp. No T.sub.comp. 4 ______________________________________ Quadrant II (A type) First Layer: Second Layer: RE Rich TM Rich ______________________________________ 5 T.sub.comp. No T.sub.comp. 3 6 No T.sub.comp. No T.sub.comp. 4 ______________________________________ Quadrant III (P type) First Layer: Second Layer: TM Rich TM Rich ______________________________________ 7 No T.sub.Comp. No T.sub.comp. 4 ______________________________________ Quandrant IV (A type) First Layer: Second Layer: TM Rich RE Rich ______________________________________ 8 No T.sub.comp. T.sub.comp. 2 9 No T.sub.comp. No T.sub.comp. 4 ______________________________________
In general, as considered from a direction perpendicular to a magnetic layer plane, spiral or concentrical tracks for recording information are formed on a disk, and a separation zone is present between adjacent tracks.
In the manufacture of a medium, directions of magnetization of magnetic layer portions located in the separation zones are often nonuniform. When over-write recording is carried out, since it is generally impossible to focus a magnetic field onto a narrow region as small as a track width, an initial field Hini. is applied to the separation zones located on two sides of each track, and directions of magnetization of the reference layer portions in the separation zones are aligned along the direction of the initial field Hini. Thus, in a portion where the direction of magnetization of the recording layer in the separation zone is unstable with respect to the reference layer, a magnetic wall is formed between the recording layer and the reference layer. If the initial field Hini. is carelessly applied even though the directions of magnetization are not nonuniform, a magnetic wall may be formed in the entire region of the separation zone.
When recording is performed according to the basic invention, a C/N ratio may be decreased or previous information may be reproduced due to the above-mentioned causes, and an information bit error rate may be undesirably increased.
The present inventors have made extensive studies, and found that when a magnetic wall is present between the recording layer and the reference layer in the separation zone, the above-mentioned problems are posed.
It is an object of the present invention to provide a method of processing a medium so as to decrease an information bit error rate during reproduction of information.